It is known to produce cyanogen chloride (see Huemer German Pat. No. 827,358, Huemer German Pat. No. 842,067 and Ullmann's Encyclop/a/ die der technischen Chemie, 3rd edition, Vol. 5 (1954) page 624) by reacting hydrogen cyanide and chlorine in the presence of water to form cyanogen chloride and hydrochloric acid to drive off the cyanogen chloride from the aqueous solution by heating, to dry over calcium chloride and to trimerize to cyanuric chloride in a subsequent reactor at 200.degree.-500.degree. C. in the presence of activated carbon. The entire disclosure of the two German patents and the page from Ullmann are hereby incorporated by reference and relied upon.
The cyanuric chloride vapors leaving the trimerization reactor go to a cooled separator from which the cyanuric chloride is discharged in crystalline form. The waste gases, which chiefly consist of unreacted cyanogen chloride, as well as chlorine, hydrogen chloride and inert gases such as phosgene or carbon tetrachloride, are washed with water in countercurrent flow in one or more columns and then pass into the open air, while the cyanogen chloride and chlorine containing aqueous solution is led back into the chlorination portion.
The hydrogen chloride formed as byproduct in the chlorination of hydrogen cyanide is withdrawn from the plant in the form of a dilute aqueous hydrochloric acid, while the water withdrawn in the form of hydrochloric acid is replaced again by the addition of fresh water. The supply of fresh water is so regulated that there always can be recovered an about 12 weight % solution of aqueous hydrochloric acid. Hydrochloric acid concentrations above 12 weight % to be sure can be obtained, however, simultaneously there would be greatly increased the saponification of the first dissolved cyanogen chloride with formation of ammonium chloride which remains in the hydrochloric acid, and carbon dioxide which passes with the cyanogen chloride into the trimerization reactor. It has likewise been shown that even at a hydrocloride acid concentration of 13 weight % there already is a trifling saponification of the cyanogen chloride to the degree of about 2%.
The trimerization of the dried cyanogen chloride is especially advantageously carried out in the presence of about 0.5-3 weight % of chlorine. However, since the trimerization of the cynanogen chloride only progresses to an extent of about 98-99% there is obtained a residual gas after the cooled separatory chamber, which according to the reaction conditions contains about 20-50 weight % cyanogen chloride, 30-70 weight % chlorine and 10-30 weight % carbon dioxide.
Because of the good solubility of cyanogen chloride in water it is readily possible to wash out the cyanogen chloride from the residual gas by a corresponding countercurrent absorption with water, whereby the aqueous solution formed which contains chlorine and cyanogen chloride, again can be recycled into the chlorination portion.
However, on account of the low solubility of chlorine in water and on account of the proportionally high content of carbon dioxide which appears as carrier gas in the absorption in water and thus opposes a complete absorption of cyanogen chloride and chlorine, the absorption of chlorine in water in insufficient. Therefore residual gas leaving the absorption column still contains 15-70 weight % of chlorine, besides 30-85 weight % of carbon dioxide and nitrogen.
If it is desired to wash out the chlorine by water, then such a large amount of water is needed that in a recycling into the chlorination portion there would occur necessarily a diluted hydrochloric acid of a type, which would be uneconomical and must be destroyed by neutralization. However, this leads to an unpermittedly high salt fraction in the waste water.
However, for environmentally protective reasons the untreated residual gas cannot be given off directly to the atmosphere. Therefore a known method treats the residual gas with aqueous alkalis, as e.g. soda lye. However thereby there is formed an aqueous solution which contains besides hypochlorite also alkali chloride and carbonate and only after reduction of the hypochlorite and neutralization can be delivered to the waste water.
To be sure the mentioned methods permit the production of a pure outgoing air; however, it is uneconomical to a high degree and loads the waste water with high amounts of salts, i.e. it converts the outgoing air problem to a waste water problem.